Selective adherence of stent-graft coverings

ABSTRACT

A method for making a radially expandable stent-graft, including positioning a radially expandable stent member concentrically over a first polymeric member, locating a second polymeric member concentrically over the stent member and first polymeric member, and joining the first polymeric member to the second polymeric member through interstices of the stent member at selective locations to form slip planes between the first and second polymeric members. The slip planes accommodate movement of the stent between the polymeric members to facilitate compression of the stent graft to a low profile.

PRIORITY

This application is a continuation of U.S. application Ser. No.11/363,689, now U.S. Pat. No. 8,012,194, which is a continuation of U.S.application Ser. No. 10/410,589, filed Apr. 9, 2003, now U.S. Pat. No.7,004,966, which is a continuation of U.S. application Ser. No.09/764,811, filed Jan. 16, 2001, now U.S. Pat. No. 6,547,814, which is adivision of U.S. application Ser. No. 09/409,209, filed Sep. 30, 1999,now U.S. Pat. No. 6,245,099, which claims the benefit under 35 U.S.C.§119(e) to U.S. Provisional Application No. 60/102,518, filed Sep. 30,1998, each of which is incorporated by reference into this applicationas if fully set forth herein.

BACKGROUND OF THE INVENTION

The use of implantable vascular grafts comprised of PTFE is well knownin the art. These grafts are typically used to replace or repair damagedor occluded blood vessels within the body. However, if such grafts areradially expanded within a blood vessel, they will exhibit somesubsequent retraction. Further, such grafts usually require additionalmeans for anchoring the graft within the blood vessel, such as sutures,clamps, or similarly functioning elements. To minimize the retractionand eliminate the requirement for additional attachment means, thoseskilled in the art have used stents, such as those presented by Palmazin U.S. Pat. No. 4,733,665 and Gianturco in U.S. Pat. No. 4,580,568,which are incorporated by reference into this application as if fullyset forth herein, either alone or in combination with PTFE grafts.

For example, the stent described by Palmaz in U.S. Pat. No. 4,733,665can be used to repair an occluded blood vessel. The stent is introducedinto the blood vessel via a balloon catheter, which is then positionedat the occluded site of the blood vessel. The balloon is then expanded,thereby expanding the overlying stent to a diameter comparable to thediameter of an unoccluded blood vessel. The balloon catheter is thendeflated and removed with the stent remaining seated within the bloodvessel because the stent shows little or no radial retraction. Use ofradially expandable stents in combination with a PTFE graft is disclosedin U.S. Pat. No. 5,078,726 to Kreamer, which teaches placing a pair ofexpandable stents within the interior ends of a prosthetic graft, havinga length that is sufficient to span the damaged section of a bloodvessel. The stents are then expanded to secure the graft to the bloodvessel wall via a friction fit.

Although stents and stent/graft combinations have been used to provideendovascular prostheses that are capable of maintaining their fitagainst blood vessel walls, other desirable features are lacking. Forinstance, features such as increased strength and durability of theprosthesis, as well as an inert, smooth, biocompatible blood flowsurface on the luminal surface of the prosthesis and an inert, smoothbiocompatible surface on the abluminal surface of the prosthesis, areadvantageous characteristics of an implantable vascular graft. Someskilled in the art have recently addressed these desirablecharacteristics by producing strengthened and reinforced prosthesescomposed entirely of biocompatible grafts and graft layers.

For example, U.S. Pat. No. 5,048,065, issued to Weldon, et al. disclosesa reinforced graft assembly comprising a biologic or biosynthetic graftcomponent having a porous surface and a biologic or biosyntheticreinforcing sleeve which is concentrically fitted over the graftcomponent. The reinforcing sleeve includes an internal layer, anintermediate layer, and an external layer, all of which comprisebiocompatible fibers. The sleeve component functions to providecompliant reinforcement to the graft component. Further, U.S. Pat. No.5,163,951, issued to Pinchuk, et al. describes a composite vasculargraft having an inner component, an intermediate component, and an outercomponent. The inner and outer components are preferably formed ofexpanded PTFE while the intermediate component is formed of strands ofbiocompatible synthetic material having a melting point lower than thematerial which comprises the inner and outer components.

Another reinforced vascular prosthesis having enhanced compatibility andcompliance is disclosed in U.S. Pat. No. 5,354,329, issued to Whalen.This patent discloses a non-pyrogenic vascular prosthesis comprising amultilaminar tubular member having an interior stratum, a unitary medialstratum, and an exterior stratum. The medial stratum forms anexclusionary boundary between the interior and exterior strata. Oneembodiment of this prosthesis is formed entirely of silicone rubber thatcomprises different characteristics for the different strata containedwithin the graft.

The prior art also includes grafts having increased strength anddurability, which have been reinforced with stent-like members. Forexample, U.S. Pat. No. 4,731,073, issued to Robinson, discloses anarterial graft prosthesis comprising a multi-layer graft having ahelical reinforcement embedded within the wall of the graft. U.S. Pat.No. 4,969,896, issued to Shors describes an inner elastomericbiocompatible tube having a plurality of rib members spaced about theexterior surface of the inner tube, and a perforate flexiblebiocompatible wrap circumferentially disposed about, and attached to,the rib members.

Another example of a graft having reinforcing stent-like members isdisclosed in U.S. Pat. No. 5,123,917, issued to Lee, which describes anexpandable intraluminal vascular graft having an inner flexiblecylindrical tube, an outer flexible cylindrical tube concentricallyenclosing the inner tube, and a plurality of separate scaffold memberspositioned between the inner and outer tubes. Further, U.S. Pat. No.5,282,860, issued to Matsuno et al. discloses a multi-layer stentcomprising an outer resin tube having at least one flap to provide ananchoring means, an inner fluorine-based resin tube and a mechanicalreinforcing layer positioned between the inner and outer tubes.

Still another stent-containing graft is described in U.S. Pat. No.5,389,106 issued to Tower, which discloses an impermeable expandableintravascular stent including a dispensable frame and an impermeabledeformable membrane interconnecting portions of the frame to form animpermeable exterior wall. The membrane comprises a synthetic non-latex,non-vinyl polymer while the frame is comprised of a fine platinum wire.The membrane is attached to the frame by placing the frame on a mandrel,dipping the frame and the mandrel into a polymer and organic solventsolution, withdrawing the frame and mandrel from the solution, dryingthe frame and mandrel, and removing the mandrel from the polymer-coatedframe.

Microporous expanded polytetrafluoroethylene (“ePTFE”) tubes may made byany of a number of well-known methods. Expanded PTFE is frequentlyproduced by admixing particulate dry polytetrafluoroethylene resin witha liquid lubricant to form a viscous slurry. The mixture is poured intoa mold, typically a cylindrical mold, and compressed to form acylindrical billet. The billet is then ram extruded through an extrusiondie into either tubular or sheet structures, termed extrudates in theart. The extrudates consist of extruded PTFE-lubricant mixture called“wet PTFE.” Wet PTFE has a microstructure of coalesced, coherent PTFEresin particles in a highly crystalline state. Following extrusion, thewet PTFE is heated to a temperature below the flash point of thelubricant to volatilize a major fraction of the lubricant from the PTFEextrudate. The resulting PTFE extrudate without a major fraction oflubricant is known in the art as dried PTFE. The dried PTFE is theneither uniaxially, biaxially or radially expanded using appropriatemechanical apparatus known in the art. Expansion is typically carriedout at an elevated temperature, e.g., above room temperature but below327° C., the crystalline melt point of PTFE. Uniaxial, biaxial or radialexpansion of the dried PTFE causes the coalesced, coherent PTFE resin toform fibrils emanating from nodes (regions of coalesced PTFE), with thefibrils oriented parallel to the axis of expansion. Once expanded, thedried PTFE is referred to as expanded PTFE (“ePTFE”) or microporousPTFE. The ePTFE is then transferred to an oven where it is sintered bybeing heated to a temperature above 327° C., the crystalline melt pointof PTFE. During the sintering process the ePTFE is restrained againstuniaxial, biaxial or radial contraction. Sintering causes at least aportion of the crystalline PTFE to change from a crystalline state to anamorphous state. The conversion from a highly crystalline structure toone having an increased amorphous content locks the node and fibrilmicrostructure, as well as its orientation relative to the axis ofexpansion, and provides a dimensionally stable tubular or sheet materialupon cooling. Prior to the sintering step, the lubricant must be removedbecause the sintering temperature of PTFE is greater than the flashpoint of commercially available lubricants.

Sintered ePTFE articles exhibit significant resistance to furtheruniaxial, or radial expansion. This property has lead many in the art todevise techniques which entail endoluminal delivery and placement of anePTFE graft having a desired fixed diameter, followed by endoluminaldelivery and placement of an endoluminal prosthesis, such as a stent orother fixation device, to frictionally engage the endoluminal prosthesiswithin the lumen of the anatomical passageway. The Kreamer Patent, U.S.Pat. No. 5,078,726, discussed above, exemplifies such use of an ePTFEprosthetic graft. Similarly, International Publication Nos. WO95/05132and WO95/05555, filed by W.L. Gore Associates, Inc., disclose balloonexpandable prosthetic stents which have been covered on inner and outersurfaces by wrapping ePTFE sheet material about the balloon expandableprosthetic stent in its enlarged diameter, sintering the wrapped ePTFEsheet material to secure it about the stent, and crimping the assemblyto a reduced diameter for endoluminal delivery. Once positionedendoluminally, the stent-graft combination is dilated to re-expand thestent to its enlarged diameter returning the ePTFE wrapping to itsoriginal diameter.

Thus, it is well known in the prior art to provide an ePTFE coveringwhich is fabricated at the final desired endovascular diameter and isendoluminally delivered in a folded or crimped condition to reduce itsdelivery profile, then unfolded in vivo using either the spring tensionof a self-expanding, thermally induced expanding structural supportmember or a balloon catheter. However, the known ePTFE coveredendoluminal stents are often covered on only one surface of the stent,i.e., either the lumenal or abluminal wall surface of the stent. Wherethe stent is fully covered on both the luminal and abluminal wallsurfaces of the stent, the covering completely surrounds the stentelements and fills the stent interstices, thereby encapsulating thestent. Examples of patents teaching encapsulated stents andencapsulation techniques include commonly assigned patents: U.S. Pat.No. 5,749,880, U.S. Pat. No. 6,039,755, U.S. Pat. No. 6,124,523, andU.S. Pat. No. 6,451,047, each of which is incorporated into thisapplication as if fully set forth herein.

When the encapsulated stent is comprised of shape memory alloy,characteristics of the stent make it necessary to encapsulate in the“large” state and then compress the encapsulated stent for delivery. Inthis case, encapsulation either increases the device's resistance tocompression, or increases the delivery profile of the device ascompression causes the polymeric material to fold or buckle around thestent. Perhaps the most serious problem is that the folding duringcompression actually encompasses folding of the stent itself, whichunduly stresses the stent material and may result in structural failure.

In contrast to the prior art, the present invention provides a method toencapsulate a stent in ePTFE whereby the structure contains pockets orregions where the ePTFE layers are not adhered to one another allowingthe stent to contract or expand without being encumbered by ePTFE andwithout folding or stressing the stent itself.

As used herein, the following terms have the following meanings:

“Fibril” refers to a strand of PTFE material that originates from one ormore nodes and terminates at one or more nodes.

“Node” refers to the solid region within an ePTFE material at whichfibrils originate and converge.

“Internodal Distance” or “IND” refers to a distance between two adjacentnodes measured along the longitudinal axis of fibrils between the facingsurfaces of the adjacent nodes. IND is usually expressed in micrometers(μm).

“Node Length” as used herein refers to a distance measured along astraight line between the furthermost end points of a single node whichline is perpendicular to the fibrils emanating from the node.

“Nodal Elongation” as used herein refers to expansion of PTFE nodes inthe ePTFE microstructure along the Node Length.

“Longitudinal Surface” of a node as used herein refers to a nodalsurface from which fibrils emanate.

“Node Width” as used herein refers to a distance measured along astraight line, drawn parallel to the fibrils, between opposinglongitudinal surfaces of a node.

“Plastic Deformation” as used herein refers to the deformation of theePTFE microstructure under the influence of a expansive force whichdeforms and increases the Node Length and results in elastic recoil ofthe ePTFE material less than about 25%.

“Radially Expandable” as used herein to describe the present inventionrefers to a property of the ePTFE tubular member to undergo radiallyoriented Plastic Deformation mediated by Nodal Elongation.

“Structural Integrity” as used herein to describe the present inventionin terms of the ePTFE refers to a condition of the ePTFE microstructureboth pre- and post-radial deformation in which the fibrils aresubstantially free of fractures or breaks and the ePTFE material is freeof gross failures; when used to describe the entire device “StructuralIntegrity” may also include delamination of the ePTFE layers.

Endoluminal stent devices are typically categorized into two primarytypes: balloon expandable and self-expanding. Of the self-expandingtypes of endoluminal stent devices, there are two principlesub-categories: elastically self-expanding and thermally self-expanding.The balloon expandable stents are typically made of a ductile material,such as stainless steel tube, which has been machined to form a patternof openings separated by stent elements. Radial expansion is achieved byapplying a radially outwardly directed force to the lumen of a balloonexpandable stent and deforming the stent beyond its elastic limit from asmaller initial diameter to an enlarged final diameter. In this processthe slots deform into “diamond shapes.” Balloon expandable stents aretypically radially and longitudinally rigid and have limited recoilafter expansion. These stents have superior hoop strength againstcompressive forces but should this strength be overcome, the deviceswill deform and not recover.

Self-expanding stents, on the other hand, are fabricated from eitherspring metal or shape memory alloy wire which has been woven, wound orformed into a stent having interstices separated with wire stentelements. When compared to balloon-expandable stents, these devices haveless hoop strength but their inherent resiliency allows them to recoveronce a compressive force that results in deformation is removed.

Covered endoluminal stents are known in the art. Heretofore, however,the stent covering has been made of a polymeric material which hascompletely subtended the stent interstices, that is, the stent wascompletely embedded in the polymeric material. This has posed difficultyparticularly with the self-expanding stents. To preserve theirself-expanding property, all covered self-expanding stents have beencovered with a polymeric covering while the stent is in its unstraineddimensional condition (i.e., its native enlarged diameter). Yet in orderto deliver a covered stent, it must be constricted to a smaller deliverydiameter. Radial compression of a stent necessarily causes theindividual stent elements to traverse the stent interstices and passinto proximity to a laterally adjacent individual stent element, therebyoccupying the previously open interstitial space. Any polymeric materialwhich subtends or resides within the previously open interstitial spacewill necessarily be displaced, either through shearing, fracturing orotherwise responding to the narrowing of the interstitial space as thestent is compressed from its enlarged unstrained diameter to itsstrained reduced diameter. Because the struts of the stent arecompletely encapsulated, resistance of the polymer may cause folding orstressing of the struts during compression.

It was recognized, therefore, that a need has developed to provide anencapsulating covering for a stent which is permanently retained on thestent, substantially isolates the stent material from the body tissueforming the anatomical passageway or from matter within the anatomicalpassageway, and which permits the stent to deform without substantialinterference from the covering material.

It is, therefore, a primary objective of the present invention toprovide a method for encapsulating an endoluminal stent such that theencapsulating covering forms non-adhered regions which act as slipplanes or pockets to permit the individual stent elements to traverse asubstantial surface area of interstitial space between adjacent stentelements without resistance or interference from the encapsulatingcovering, thereby avoiding damage or stress to the stent elements. It isa further object of the present invention to use the pockets between thebonded regions to contain and deliver therapeutic substances. It isanother objective of the present invention to provide an apparatus forapplying to and selectively adhering sections of the encapsulatingcovering about the stent, and to provide a selectively adheredencapsulated covered stent-graft device.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention relates generally to endoluminalstent-graft devices suitable for percutaneous delivery into a bodythrough anatomical passageways to treat injured or diseased areas of thebody. More particularly, the present invention relates to a method ofbonding microporous polytetrafluoroethylene (“PTFE”) coverings over astent scaffold in a manner which maintains unbonded regions to act asslip planes or pockets to accommodate planar movement of stent elements.In one embodiment of the present invention bonded and unbonded regionsare formed by means of a mandrel which has a pattern of either raisedprojections or recesses in its surface which are either synchronous orasynchronous, respectively, with stent elements.

In one embodiment, an encapsulated stent-graft device is provided inwhich an endoluminal stent having a plurality of individual stentelements separated by interstitial spaces is circumferentially coveredalong at least a portion of its longitudinal axis by at least oneluminal and at least one abluminal covering of a polymeric material, theluminal and abluminal coverings being selectively adhered to one anotherat discrete portions thereof in a manner which forms a plurality of openpockets surrounding a plurality of stent elements. In anotherembodiment, a radially expandable reinforced vascular graft includes afirst layer of biocompatible flexible material, a second layer ofbiocompatible flexible material, and a support layer sandwiched betweenthe first and second layers of biocompatible flexible material.

In one embodiment, a selective bonding system can be advantageously usedto produce inflatable pockets by bonding the first layer to the secondlayer in defined patterns. The resulting structure can then be inflatedand stiffened by injection of a fluid resulting in a supportingstructure without inclusion of a stent. A crude analogy might be theconstruction of an air mattress that is composed of flexible polymericlayers bonded to each other in a predetermined pattern.

The at least one luminal and at least one abluminal covering of apolymeric material are preferably comprised of expanded PTFE, unexpandedporous PTFE, woven polyester or expanded PTFE yarns, polyimides,silicones, polyurethane, fluoroethylpolypropylene (FEP),polypropylfluorinated amines (PFA), or other related fluorinatedpolymers. The stent may be made of any strong material which can undergoradial expansion but which is also resistant to non-elastic collapsesuch as silver, titanium, nickel-titanium alloys, stainless steel, gold,or any suitable plastic material capable of maintaining its shape andmaterial properties at sintering temperatures and having the necessarystrength and elasticity to enable radial expansion without collapse dueto the presence of the polymer coverings.

A preferred embodiment of the radially expandable reinforced vasculardevice includes a tubular stent, having a plurality of stent elementsand stent interstices, concentrically covered along at least a portionof its longitudinal length by a luminal polymeric covering and anabluminal polymeric covering. The luminal and abluminal polymericcoverings are discontinuously joined to one another through some of thestent interstices. The luminal and abluminal polymeric coverings may beshorter in length than the stent member to permit opposing stent ends toflare outwardly upon radial expansion of the stent member.Alternatively, the ends of the stent member may be completely encased bythe luminal and abluminal polymeric coverings.

The stent member is preferably a self-expanding stent, which may beeither an elastic spring material stent, such as a stainless steel stentas disclosed in U.S. Pat. No. 5,266,073 to Wall, a non-woven stainlesssteel self-expanding stent as disclosed in U.S. Pat. No. 5,282,824 toGianturco, or a thermoelastic stent made of a shape memory alloy, e.g.,a nickel-titanium alloy commonly known as NITINOL, such as thatdisclosed in U.S. Pat. No. 5,147,370 to McNamara et al., each of whichis incorporated by reference into this application as if fully set forthherein. As used herein, a tubular shaped support member preferablycomprises a stent made of silver, titanium, stainless steel, gold, orany suitable plastic material capable of maintaining its shape andmaterial properties at sintering temperatures and having the strengthand elasticity to permit radial expansion and resist radial collapse.

In accordance with the present invention, selective bonding of expandedPTFE luminal and abluminal layers encapsulates the endoluminal stent andisolates the stent from both the tissue forming the anatomicalpassageway as well as any fluid, such as blood, bile, urine, etc. whichmay pass through the anatomical passageway. The presence of slip planesor pockets formed by the selectively adhered regions of ePTFE: i)permits freedom of movement of stent elements within the encapsulatingcovering during both expansion and contraction of the stent along eitherits radial or longitudinal axes; ii) permits uniform folding of theePTFE stent covering material which is complementary to the structure ofthe stent element lattice; iii) permits movement of the stent relativeto the ePTFE encapsulating layers; iv) reduces forces required tocompress or dilate the stent in the case of elastically or thermallyself-expanding stents; v) reduces radial expansion pressures required toballoon expand an ePTFE encapsulated stent; and vi) provides voidregions which may be used in conjunction with the microporousmicrostructure of the ePTFE covering material to retain and releasebioactive substances, such as anticoagulant drugs, anti-inflammatorydrugs, or the like.

Alternative arrangements of the stent member or other suitablestructural support sufficient to maintain the lumenal patency of thelumenal and abluminal polymer coverings may be employed. For example, aradially expandable, articulated reinforced vascular graft may be formedby concentrically interdisposing a structural support assemblycomprising multiple stent members spaced apart from one another betweentwo tubular polymer covering members, then partially joining the twotubular polymer covering members by circumferentially compressingselected regions of the two tubular polymer covering members andthermally bonding the selectively compressed regions to one another.

The present invention also encompasses selective bonding of multiplepolymeric layers to create an inflatable structure. Such a structure canbe inflated by fluids delivered through lumens within the deliverycatheter. The selective bonding method allows creation of devices withmultiple adjacent channels or pockets. Some of these pockets can bepre-filled with a therapeutic drug to prevent restenosis or localthrombosis. Alternate pockets can be arranged for fluid inflation afterthe device is inserted.

One method of making the foregoing encapsulated stent-graft is to joinconcentrically a luminal polymeric tube, an endoluminal stent, and anabluminal polymeric tube and to place the assembly onto a mandrel havinga plurality of raised projections separated by land areas, or by aplurality of land areas separated by a plurality of recesses. Either theraised projections or the land areas are patterned to match a pattern ofeither the stent elements of the stent interstices, both the stentelements and stent interstices or portions of each. In this way theprojections or the landed areas exert pressure, respectively on selectregions of the PTFE resulting in limited regions of adherence or fusionwhen the device is heated to sintering temperatures. Using a mandrel,luminal pressure is selectively applied to produce selectively placedbonds. As will become clear, bonding pressure can be applied from theluminal or the abluminal or both surfaces of the device.

The present invention is also directed to a process for making aradially expandable reinforced stent-graft device by: a) positioning aradially expandable stent member including a plurality of interconnectedstent elements and a plurality of interstices between adjacentinterconnected stent elements, concentrically over a first polymericcover member; b) positioning a second polymer cover memberconcentrically over the radially expandable stent member and the firstpolymeric cover member; c) selectively joining portions of the firstpolymeric cover member and the second polymeric cover member through aplurality of the interstices of the stent member, while leaving portionsof the first and second polymeric cover members unjoined and formingslip planes or pockets to accommodate movement of at least a portion ofthe interconnected stent elements therethrough; and d) fully joiningopposing end regions of the first and second polymer cover membersthrough the interstices of the stent member proximate to opposing endsof the stent member.

Fixing the support layer to the biocompatible graft layers includesselectively applying pressure to the portions of the luminal andabluminal polymer covers after they are loaded onto a mandrel and thenheating the resulting assembly at sintering temperatures to form amechanical bond at the selected areas of applied pressure.Alternatively, a pattern of at least one of an adhesive, an aqueousdispersion of polytetrafluoroethylene, a polytetrafluoroethylene tape,fluoroethylpolypropylene (FEP), or tetrafluoroethylene (collectively the“adhesive”) may be introduced between the luminal and abluminal polymercovers at selected positions, followed by heating the assembly to themelt temperature of the adhesive to bond the luminal and abluminalpolymer covers while leaving unbonded slip plane regions to accommodatemovement of the stent elements. If ultraviolet curable adhesives areused, a UV laser or a photolithography system can be used to create thebond pattern. Many thermoplastic polymers such as polyethylene,polypropylene, polyurethane and polyethylene terephthalate can also beused. If pieces of one of these or similar polymers are placed orattached to one of the polymer covers in the region to be bonded, heatand pressure will melt the thermoplastic causing it to flow into thepores of the ePTFE, thereby bonding the ePTFE layers together.

These and other embodiments, features and advantages will become moreapparent to those skilled in the art when taken with reference to thefollowing more detailed description of the invention in conjunction withthe accompanying drawings that are first briefly described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a is a process flow diagram illustrating a preferred method ofmaking a stent-graft device

FIG. 2 is a is a perspective view of a mandrel having longitudinalridges or splines.

FIG. 3 is a is a cross-section view of the mandrel shown in FIG. 2.

FIG. 4 is a perspective view of a stent-graft device illustratingselected regions of bonding between the luminal and abluminal stentcovers and a plurality of slip plane pockets intermediate the luminaland abluminal stent covers.

FIG. 5 is a is a cross-sectional view taken along line 5-5 of FIG. 4.

FIG. 6 is a scanning electron micrograph illustrating a selectivelybonded region and a slip plane pocket with a stent element residingtherein.

FIG. 7 is a is a perspective view of a mandrel having circumferentialridges (as opposed to longitudinal splines).

FIG. 8 is a is a flow diagram showing a method of using adhesives tocreate selective adherence.

FIG. 9 is a flow diagram of an alternative method of using adhesives tocreate selective bonds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are identicallynumbered. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. The detailed description illustrates by way of example, notby way of limitation, the principles of the invention. This descriptionwill clearly enable one skilled in the art to make and use theinvention, and describes several embodiments, adaptations, variations,alternatives and uses of the invention, including what is presentlybelieved to be the best mode of carrying out the invention.

The selective adherence encapsulation of the present invention is animprovement to the total adherence method taught in U.S. Pat. No.5,749,880 that is incorporated by reference into this application as iffully set forth herein. The '880 patent discloses a method forencapsulating a support stent by placing the stent over a first tubularmember of unsintered ePTFE and then placing a second tubular member ofunsintered ePTFE coaxially over the stent so that the stent issandwiched between two layers of ePTFE. Radial force is applied eitherinternally or externally to force the first tubular member into contactwith the stent and into contact with the second tubular members throughopenings in the stent or, respectively, to force the second tubular intocontact with the stent and into contact with the first tubular memberthrough openings in the stent. Finally, the compound structure isexposed to an elevated temperature to bond the first tubular member tothe second tubular member wherever they are pressed into contact. In oneembodiment an adhesive spread between the tubular members achieves thebonding. In a preferred embodiment the elevated temperature is asintering temperature (above the crystalline melting point of PTFE) anddirect PTFE to PTFE bonds form.

As mentioned above, a potential drawback of this approach is that whenthe radial dimensions of the stent change, movement of components of thestent (necessary for radial dimensional changes) may be impeded bysurrounding ePTFE. If the stent is encapsulated in an expanded form andthen reduced in diameter prior to insertion into a patient, theencapsulating ePTFE may significantly increase the force needed tocompress the stent and may fold in a manner so as to increase theprofile of the collapsed device. If the bonding of the first member tothe second member is selective, i.e., does not occur through allavailable openings in the stent, slip planes or pockets will be left inthe structure so that stent components can reorient within these pocketswithout encountering resistance from the ePTFE. Without the slip planesformed by the selective bonds of the present invention, crimping a shapememory stent may cause the stent members to fold or otherwise becomestressed. This can result in permanent damage to the stent.

There is a considerable possible range of extent for the selectiveadherence of the instant invention. At one extreme is a fullyencapsulated stent as provided by the '880 patent in which there is fullbonding between all areas of the two tubular members in which the stentstruts do not block contact. At the other extreme would be a “spotwelded” device where only tiny areas, probably in the middle of the openareas of the stent structure, are bonded. At that extreme, there mightbe a tendency for the PTFE members to separate from the stent should thespot weld bond strength be exceeded; however, the spot weld structurewould provide virtually no impedance to radial deformation of the stent.

The optimum extent of selective adherence, as well as the geometricposition of the bonds in relation to the stent, depends on the structureof the stent as well as the desired properties of the completed device.Complete control of the bond positions can be achieved by a numericallycontrolled (NC) machine in which the two-ePTFE members with theinterposed stent are mounted on a mandrel that is attached to thespindle drive of a modified NC lathe. In this device a heated tool whosetip is equal to the desired spot weld area is automatically pressed ontothe mandrel-mounted ePTFE-stent sandwich in proper registration tocreate a bond in an open region between components or struts of thestent. The tool moves away slightly as the mandrel turns to exposeanother open region and the tool then moves in to create a second bondand so on. Depending on the distance that the mandrel turns, the spotwelds may be in adjacent open spaces or may skip one or more openspaces. As the mandrel is turned, the tool advances along thelongitudinal axis of the mandrel so that virtually any patterns of spotwelds can be created on the ePTFE-stent device. The precise pattern isunder computer control and an entire stent can be treated quite quickly.If the design calls for spot welds of different surface areas, the stentcan be treated with different tools (e.g., different areas) in severalpasses. An ultrasonic welding tip can readily be substituted for theheated tool. It is also possible to use radiant energy, as with a laser,to effect similar results. However, the inventors presently believe thatpressure as well as heat are needed for the best bonds. Currently,laser-induced bonds do not appear to be as strong as bonds that are madewith heat and pressure unless a curable adhesive system (as with a UVlaser) is employed.

Splined or textured mandrels can also be used to apply selective heatand pressure to create selective adherence between the ePTFE members. By“spline or splined” is meant a cylindrical structure with longitudinallyoriented ridges equally spaced about the structure's circumference.Wherever the first and second ePTFE tubular members come into contact abond can be formed if heat and pressure are applied. If the ePTFEtubular members and support stent are placed over a mandrel whosesurface is patterned with elevated and depressed regions (hills andvalleys), the elevated regions or ridges will apply pressure to theoverlying stent-ePTFE regions allowing selective bonding of thoseregions. Regions of ePTFE overlying valleys will not be pressed togetherand no bond will form there. That is, the pattern of the mandrel will betranslated into an identical pattern of bonded regions in thestent-graft device. To make this translation, the process diagram ofFIG. 1 is followed, as described below.

In a first step 32, a first ePTFE tubular member is placed on a mandrel.Preferably, the first tubular member is composed of unsintered ePTFE. Ina second step 34, a stent device is placed over the first tubularmember. In a third step 36, a second ePTFE tubular member is slidcoaxially over the stent. The second tubular member may be unsintered orpartially sintered. Use of a partially sintered second tubular memberreduces the chance of tearing the member while pulling it over thestent. It will be apparent to one of skill in the art that there is anadvantage to using a second tubular member with a slightly largerdiameter than the first tubular member. However, if the second tubularmember is too large, folds or creases may develop during the bondingprocess.

This entire process may use one of the textured mandrels that will bedescribed below. However, it is also possible to assemble one or bothtubular members and the stent on a smooth mandrel and then slip theassembly off the smooth mandrel and onto the textured mandrel. If thefit is fairly tight, it may be easier to place the stent over the firsttubular member when that member is supported by a smooth mandrel. Also,there may be a limited number of textured mandrels available forproduction so that making a number of ePTFE-stent assemblies on lessexpensive smooth mandrels may result in a significant savings of time.If a smooth mandrel is used, the stent assembly is transferred to atextured mandrel before the next step (wrapping) occurs.

In a fourth step 38, the ePTFE-stent assembly is helically wrapped withPTFE “tape.” This tape is actually a long, thin strip of PTFE of thetype generally known as “plumber's tape.” The tape is evenly wound overthe stent device so that the device is covered from end to end. The tapeis wound so that the long axis of the tape is approximately normal(offset by 10-15°) to the long axis of the stent device. Ideally, thereshould be some overlap of the tape covering the device so that coverageis even and complete. In fact an overlap ratio wherein five revolutionsis needed to progress one tape width has proven effective. The tapeshould be applied with a controlled and even tension so that it issufficiently tight to apply pressure at right angles to the surface ofthe stent device. One way of achieving this is to use a force clutch onthe tape spool to ensure a reproducible tension in the tape as it iswound over the stent device. While this process can be performed byhand, it is fairly easy to automate the winding process by having themandrel mounted in a modified lathe. As the lathe spindle turns, thespool of tape automatically advances along the turning mandrel ensuringan even and reproducible wrapping.

In a fifth step 42, the wrapped assembly is then placed into an oven ata temperature above or nearly equal to the crystalline meltingtemperature of ePTFE. The wrapping applies pressure to regions of ePTFEthat are underlaid by raised portions of the textured mandrel. The ovenprovides the necessary heat to cause a strong ePTFE-ePTFE bond to formin these regions. The sintering time can vary from a few minutes to afew tens of minutes. The overall time depends to some extent on the massof the mandrel. If the mandrel is solid, it may take a considerable timefor the surface of the mandrel to reach sintering temperatures. Theprocess can be speeded up by using a hollow mandrel or even a mandrelcontaining a heating element so that the ePTFE is rapidly brought to asintering temperature. A thermistor or similar temperature sensor isadvantageous embedded into the surface of the mandrel so that it ispossible to determine when the ePTFE reaches sintering temperature. Inthis way the process can be accurately timed.

In the final step 44, the tape is removed from the mandrel (aftercooling) and the finished device is removed. Results in this stepindicate the success of the sintering step 42. If sintering time ortemperature is excessive, there may be some bonding of the PTFE tape tothe stent device. The solution is to reduce the sintering time and/ortemperature in future sintering. This is one reason that time,temperature and wrapping force should be carefully controlled. Thisproblem can also be avoided by using means other than PTFE wrapping toapply pressure to the device during the sintering process. At firstglance it would appear that the radial pressure can be applied by a“clam shell” heating device that clamps around the stent device andmandrel. However, such a device is not capable of applying even radialpressure. One possible solution is to divide the clam shell device intoa number of segments, preferably at least six, each of which is equippedwith pressure means to force the segment radially towards the center oftextured mandrel. Similarly, the mandrel can be divided into segments orotherwise be capable of an increase in diameter (e.g., by formation froma material having a large coefficient of expansion upon temperatureincrease) to create radial pressure between the surface of the mandreland the surrounding clam shell device.

An additional method of achieving bond pressure without wrapping is touse a clamshell device having an inner surface relief mirroring thetextured mandrel. That is, there would be ridges and valleys that wouldexactly register with the ridges and valleys on the mandrel when theshell is closed. Similarly, a flat surface could be provided with ridgesand valleys matching the mandrel surface if that surface were unrolledonto a flat plane. With such a surface it is possible to roll themandrel in contact and registration with the flat pattern so thatdefined pressure is applied to the raised mandrel regions. The downwardforce applied to the mandrel controls the bond pressure while the rateof rolling controls the time a given bond is under pressure. Thisprocess can be carried out in an oven or the mandrel and surface cancontain heating elements. One method of ensuring registration betweenthe mandrel pattern and the flat surface pattern is to have gearsattached to one or both ends of the mandrel mesh with a toothed rackthat runs along one or both edges of the patterned surface. Contactpressure is controlled by weight of the mandrel or by a mechanicallinkage that applies a controlled downward force to the mandrel.

To this point no mandrel patterns or textures have been described. Itwill be clear to one skilled in the art that this invention permits acomplex pattern wherein the entire stent structure is mirrored by thevalleys and ridges of the mandrel with the structural members of thestent fitting into the valleys and the apices of the ridges or raisedportions falling at discrete points within the open areas of the stent.What may be somewhat less obvious is that far simpler patterns can alsoproduce excellent results in the present invention. One simple mandreldesign is a “splined” mandrel wherein the mandrel has a number oflongitudinal ridges (splines) so that a cross-section of the mandrellooks something like a toothed gear. FIG. 2 shows a perspective view ofsuch a mandrel 20 with longitudinal splines 22. FIG. 3 shows a crosssection of the mandrel 20 wherein it is apparent that the splines 22have rounded edges to avoid damaging or cutting the surface of theePTFE.

FIG. 4 shows a perspective view of an encapsulated stent 30 made on thesplined mandrel 20. The stent 46 is composed of struts 48 arranged in adiamond pattern. Regions 52 at the ends of the device (marked bycross-hatching) have complete bonding between the two-ePTFE tubularmembers. This region is produced by smooth, non-splined regions of themandrel. Dotted lines 54 mark the position of the splines and theresulting regions of selective bonding. That is, the device has spacedapart bonded regions running the length of the open diamond regions 56.Because of this orientation, successive tiers of diamond regions 56along the longitudinal axis of the device are alternately bonded andunbonded. FIG. 6 shows a scanning electron micrograph of an obliquesection through a longitudinally selectively bonded stent 44. Across-section of the strut 48 is shown as well as a bonded region 54 andan unbonded slip pocket 62. The unbonded pockets 62 allow free movementof the stent struts 48. However, even those diamond regions 56containing bonds 54 allow relatively unimpeded movement of the struts 48because the bond 54 is only down the central part of the diamond region56—relatively distant from the struts 48. Tests show that theselectively bonded stent 30 can be radially compressed with considerablyless force than a stent that is encapsulated by uniformly bonding allregions where the ePTFE tubular members contact each other. Thelongitudinal bonds somewhat restrict longitudinal compression of thedevice as the bonded regions buckle less readily than unbonded ePTFE.

The longitudinal bonds 54 do restrict the side to side flexibility orbendability of the device to some extent. In some applications thisstiffening of the device is desirable while in other applications oneneeds a stent device that is able to bend more freely. Increased lateralflexibility can be achieved by using a mandrel with radial ridges ratherthan longitudinal ridges as shown in FIG. 7. Again the ridges 58 arespaced apart in relation to the strut 48 spacing in the stent to beencapsulated. If the stent 46 shown in FIG. 4 is used, the radial ridges58 can be spaced apart to place circumferential bonds through alternatetiers of diamond regions 56. The resulting device is more bendablelaterally than the version with longitudinal bonds. In addition, thecircumferential bonds result in a device that is more easily compressedlongitudinally.

It is clear that the area and orientation of the bond regions influencethe properties of the final device. For example, a helical pattern ofridges produces a device with intermediate properties: it is morelaterally bendable that the longitudinally bonded device of FIG. 4, butit has more resistance to longitudinal compression than does a devicewith circumferential bonds. The pitch of the helical pattern controlsthe overall effect with shallow pitches acting more like circumferentialridges and steep pitches acting more like longitudinal ridges. Multiplehelices can be used with opposing (e.g., clockwise and counterclockwise) ridges, producing a device that is more resistant to lateralbending. Virtually any combination of the described patterns can be usedto produce devices having a preferred direction of bendability ordevices that resist longitudinal compression in one region whilepermitting such compression in another.

The stent device illustrated in the figures is one in which the stentstruts form courses or diamond-shaped spaces and the struts continuefrom course to course to create an extended tubular device. Stents arealso available which consist of only a single course (or segment) ofdiamond-shapes. The current method can advantageously be used to combinea number of these segments together to make an extended tubular device.Frequently, these single segment stents consist of an alternation oflarger and smaller diamond shapes. For example, the segments can bearranged with large diamonds touching large diamonds. Other arrangementsincluded a “twisted” design wherein each successive segment isrotationally offset and an “alternating” design wherein alternatesegment are rotated so that a given large diamond is bounded on eitherside by a small diamond. The precise properties of the resultingencapsulated device depend on these factors. However, the significantthing about the prior art encapsulation is that it produced a devicethat is relatively stiff and unbending.

Various adhesives (as opposed to directly adhering PTFE to PTFE) canalso be used to create the pattern of bonded regions. FIG. 8 shows adiagram of one method for using adhesives to create selective bonds. Ina first step 32, a tubular graft member is placed on a support such as amandrel. In a second step 34, a stent (or stents) is placed over thefirst graft member. In a third step 64, a coating of adhesive is placedover the stent graft combination. This adhesive is one that is“activatable” meaning that the material is not inherently sticky as itis applied. However, it can be activated by applying heat, light or someother energy so that it hardens or otherwise changes to form a permanentbond. In the next step 64, a second tubular member is placed over theadhesive-coated stent. In the final step 66, a pattern of desired bondsis inscribed on the device with, for example, a laser or a heated probeor a photolithographic mask image. The inscribing process providesenergy to local regions of the structure to activate the adhesive andcreate selectively bonded regions. A number of different activatableadhesive materials can be used in the present invention. One suchmaterial might be a layer or coating of a thermoplastic such aspolyethylene. This material can be activated by heat that melts it sothat it flows into the pores of the ePTFE. After cooling, the plastichardens so that the PTFE of one tubular member is bonded to the othertubular member.

FIG. 9 shows a second adhesive-based method of creating selective bonds.The initial steps are the same as in the previous method. However, instep 68 the adhesive material is applied selectively to form the futurepattern. This can be done, for example, by a screening or offsetprinting method. An inherently sticky adhesive can be used or anactivatable adhesive (as in the previous method) can be employed. Thesecond tubular member is applied (step 36) and the adhesive pattern isformed either by applying pressure (when using an inherently stickyadhesive) or by applying pressure followed by an activation step (forexample, heating to melt a thermoplastic adhesive).

This invention has been described and specific examples of the inventionhave been portrayed. While the invention has been described in terms ofparticular variations and illustrative figures, those of ordinary skillin the art will recognize that the invention is not limited to thevariations or figures described. In addition, where methods and stepsdescribed above indicate certain events occurring in certain order,those of ordinary skill in the art will recognize that the ordering ofcertain steps may be modified and that such modifications are inaccordance with the variations of the invention. Additionally, certainof the steps may be performed concurrently in a parallel process whenpossible, as well as performed sequentially as described above.Therefore, to the extent there are variations of the invention, whichare within the spirit of the disclosure or equivalent to the inventionsfound in the claims, it is the intent that this patent will cover thosevariations as well. Finally, all publications and patent applicationscited in this specification are herein incorporated by reference intheir entirety as if each individual publication or patent applicationwere specifically and individually put forth herein.

1. A method for making a radially expandable stent-graft, comprising:positioning a radially expandable stent member concentrically over afirst polymeric member, the stent member including a plurality ofinterconnected stent elements forming a plurality of intersticestherebetween; locating a second polymeric member concentrically over thestent member and first polymeric member; joining the first polymericmember to the second polymeric member through the interstices of thestent member at selective locations to form slip planes between thefirst and second polymeric members to accommodate movement of at leastsome of the stent elements; and fusing the first polymeric member to thesecond polymeric member through the interstices of the stent member atboth a first and second end region thereof, a majority of the area ofthe first and second end region interstices occupied by the fused firstand second polymeric members.
 2. The method according to claim 1,wherein the joining and fusing steps include first applying pressure,and then applying heat.
 3. The method according to claim 2, whereinapplying pressure includes helically wrapping the stent-graft with PTFE.4. The method according to claim 1, further comprising positioning thefirst polymeric member and stent member over a mandrel having a patternof elevated and depressed regions.
 5. The method according to claim 4,wherein the joining and fusing steps include inserting the stent-graftinto a heating device having an inner surface that minors the mandrelpattern to register with the elevated and depressed regions when theheating device is closed.
 6. The method according to claim 1, whereinthe first polymeric member is a tubular member of unsintered ePTFE. 7.The method according to claim 6, wherein the second polymeric member isa tubular member of unsintered or partially sintered ePTFE.
 8. Themethod according to claim 1, wherein the selective locations form ahelical pattern about the stent-graft.
 9. The method according to claim8, wherein the selective locations form two or more helical patternsabout the stent-graft.
 10. The method according to claim 1, wherein thepositioning step further comprises applying an activatable adhesivecoating to the stent member and first polymeric member.
 11. The methodaccording to claim 10, wherein the joining step comprises an inscribingprocess that provides energy to the selective locations to activate theadhesive.
 12. The method according to claim 10, wherein the activatableadhesive is applied in a pattern to enable the joining at selectivelocations.